Minnesota's Wet Precipitation Regime at the End of the 20th Century

(prepared August 1999)

Precipitation over much of Minnesota during 1999 has followed a pattern of wetness observed repeatedly in many parts of the state during the 1990's. The precipitation ranking map for January 1, 1999 through August 16, 1999 shows that very wet conditions prevailed in northern and southeastern Minnesota. Values at the 99th percentile indicate that precipitation totals for the period were near all-time record levels when compared with the long-term data.

Across Minnesota, precipitation in the 1990's exceeded the climatological benchmark (1961-1990 normal) by a significant amount. The cumulative departure from normal map for the period January 1, 1991 through August 16, 1999 shows that many regions of the state exceeded the historical average by more than 30 inches. In some areas of northwestern, south central, and southeastern Minnesota, the aggregate departure exceeds 40 inches. In northwestern Minnesota, a 40 inch positive departure is the equivalent of receiving an additional two years of annual average precipitation. The 1991 start date was chosen because 1991 marked the end of the 1987 - 1990 drought in northwestern Minnesota. In southern Minnesota, the drought concluded in 1990.

It is useful to evaluate the 1990's precipitation data for multi-year intervals. Multi-year totals were compared with other whole year periods of the same duration over the period of record (1891 - 1990). The ranking map for the four-year period 1995 - 1998 highlights the wet anomaly in northwestern Minnesota, where high water levels remain a problem. The eight-year interval 1991 - 1998 includes the 1993 deluge. We see that for the 1991 – 1998 period, large areas of Minnesota were above the 90th percentile, and a significant portion of the state ranked in the 99th percentile. The 1991 - 1998 precipitation totals; the 1999 wetness; and for some, a very wet 1990; will combine to position the 1990's as the wettest decade of the 20th century in many Minnesota communities. To view a year-by-year breakdown of annual precipitation totals, departures from normal, and historical ranking for the 1990's; see the Annual Precipitation Maps.

Many will recall that the period 1977 - 1986 was also very wet when compared with the long-term data. Therefore, in spite of the 1976 drought and the three to four-year drought of the late 1980s, the last 25 years were quite wet when compared with the first three quarters of the 20th century. An animation depicting the historical ranking of running 5-year totals for the period 1981 - 1998 shows the heavy precipitation patterns in the early to mid-1980's, returning again in the 1990's.

Individuals who study and manage Minnesota's water resources must note that the current state of the climate is just one of a variety of possible outcomes. Climate extremes should not be considered aberrations, but rather treated as an inherent component of a continental climate. The present-day relative abundance of water is uncommon, but spells of wet weather are not without precedence. Nor is it without precedent for the climate state to change rapidly between wet and dry regimes.